1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device, and in particular to a method for setting a pre-tilt angle of liquid crystal molecules contained in the liquid crystal display device.
2. The Related Arts
A liquid crystal display device is a kind of displaying device, which comprises a liquid crystal material enclosed between two opposite substrates. The displaying device effects light switching with optical anisotropy of the liquid crystal molecule by applying electrical excitation. Also, with the anisotropy of refractivity of liquid crystal molecule, application of electrical voltage to the liquid crystal molecule is made to reorient the axis of anisotropy of refractivity in order to control the of light intensity transmitting through the liquid crystal molecule for achieving control of luminance. Whether a liquid crystal display device is good or not is primarily determined by the liquid crystal panel thereof, for the quality of the liquid crystal panel directly affects the viewing of the image. Further, the liquid crystal panel takes more half of the cost of the entire device, making it the primary factor that affecting the cost of the liquid crystal display device. Apparently, to make a good selection of liquid crystal display device, the first factor to be taken into account is the liquid crystal panel thereof. The liquid crystal panel is generally made of two glass substrates and a liquid crystal material that is arranged between the glass substrates and does not gives off light by itself and thus requires illumination by a backlight source.
Driving liquid crystal is often achieved with two techniques, namely passive matrix driving and active matrix driving. With the increasing demand of higher resolution, an active matrix display mode using thin-film transistor (TFT) is currently the primary mode of liquid crystal displaying. In a liquid crystal display incorporating such a thin-film transistor structure, when a voltage is applied to the liquid crystal molecules while irradiating the liquid crystal molecules with ultraviolet light, it is actually to set the pre-tilt angle of the liquid crystal molecule by applying a TFT conduction voltage to each gate line and a required voltage to each data line and at the same time exposing the liquid crystal molecule to the irradiation of ultraviolet.
As mentioned above, setting the pre-tilt angle of the liquid crystal molecule is done with the irradiation of ultraviolet light, where the liquid crystal material contained in a liquid crystal cell comprises photo- or heat-polymerizable monomers. To have the photo- or heat-polymerizable monomers completely polymerized, when a driving voltage is applied to the liquid crystal panel material that contains the photo- or heat-polymerizable monomers (the driving voltage being applied to cause a small-range rotation of liquid crystal molecules in order to set the pre-tilt angle of the liquid crystal molecule in advance), a light that comprises an intense ultraviolet component is irradiated on the liquid crystal cell. However, increasing the intensity of the irradiation light or the intensity of the ultraviolet light contained in the irradiation light will cause deterioration of transmittance of the liquid crystal material, leading to an issue of lowering of contrast. To avoid the issue of deterioration of contrast caused by lowered transmittance, irradiation may be made with low intensity ultraviolet light but for an extended period for sufficiently polymerizing the photo- or heat-polymerizable monomers. However, this process inevitably lowers down the manufacturing efficiency of liquid crystal display devices.
FIG. 1 is a view showing the principle of setting pre-tilt angle of liquid crystal molecule contained in a photo- or heat-polymerizable monomer based liquid crystal material. In the drawing, without application of driving voltage, the liquid crystal molecules 100 are aligned perpendicularly between two substrates 300. When a driving voltage is applied, the liquid crystal molecules 100 are rotated by 0-90°. To increase the response speed of a liquid crystal display device, the liquid crystal molecules 100 are set with a pre-tilt angle, of which the size is controlled within 2°. Under the condition that a voltage is applied to have the liquid crystal molecules 100 rotated to the pre-tilt angle, irradiation of light comprising ultraviolet component or ultraviolet light 400 is made on the photo- or heat-polymerizable monomers 200 causes the photo- or heat-polymerizable monomers 200 to polymerize so as to set the pre-tilt angle for constraining the alignment direction of the liquid crystal molecules 100.
FIG. 2 is a plot illustrating image sticking rate varying with exposure of ultraviolet light. The plot shows that the image sticking rate is related to the amount of exposure. When exposure is relatively small, sufficiently polymerizing the monomers may not be possible and thus, no strong cross-linked structure can be obtained. Consequently, the probability of image sticking is increased.
FIG. 3 is schematic view showing some known solutions of ultraviolet light irradiation, including (1) applying driving voltage while irradiating the light; (2) first irradiating the light and at the same time applying driving voltage and then removing the driving voltage with only irradiation of the light; (3) first irradiating the light and then applying driving voltage while irradiating the light; and (4) first irradiating the light and the applying driving voltage while irradiating the light, and then removing the driving voltage. In the drawing, in irradiating a liquid crystal material that contains photo- or heat-polymerizable monomers with a light comprising ultraviolet component of predetermined intensity or ultraviolet, a voltage that causes the liquid crystal molecules to rotate to the pre-tilt angle is applied, and afterwards, a light that comprises an ultraviolet component or ultraviolet light having other intensity is used to irradiate the liquid crystal cell. However, when the liquid crystal material is irradiated with light without application of driving voltage, since no external driving exists, the liquid crystal molecules may deviates from the pre-tilt angle. Under this condition, some of the polymers that have already been polymerized imposes a limitation to the liquid crystal molecules that leads to difference of light transmittance in the liquid crystal material located in this area, leading to difference of contrast.
Chinese Patent Application No. 02819491.8 proposes a solution of irradiation for setting pre-tilt angle of liquid crystal molecule with irradiation of light:
(1) Before a step of setting pre-tilt angle for liquid crystal molecules, a voltage Vx that is greater than threshold voltage for rotation of liquid crystal molecules and smaller than saturation voltage for rotation of liquid crystal molecules is applied. Afterwards, the voltage Vx is changed to V and during the application of voltage V, a light comprising an ultraviolet component or an ultraviolet light is applied to irradiate the above liquid crystal material to polymerize the photo- or heat-polymerizable monomers so as to constrain the pre-tilt angle of the liquid crystal molecules.
(2) After the step of setting pre-tilt angle through polymerization of the photo- or heat-polymerizable monomers, irradiating light comprising ultraviolet component or ultraviolet light on the liquid crystal material under a condition of applying a voltage that does not cause rotation of the liquid crystal molecules or applying no voltage at all.
(3) In the step of irradiating the photo- or heat-polymerizable monomers with the light comprising ultraviolet component or ultraviolet light, the step is repeated for at least twice.
Shortcomings of the above method are as follows. The number of the step of making the photo- or heat-polymerizable monomers of the liquid crystal material polymerized is large and each of these steps requires different components and intensity of the irradiation light. Further, a sharp boundary is present between these steps and the parameters of each step are set different, such as, the time when the driving voltage is applied in these steps, variation of light intensity among the steps or the variation of intensity of ultraviolet light among these steps.
Further, under the condition of irradiation with light comprises ultraviolet component or ultraviolet light with application of no driving voltage or a voltage that does not cause rotation of liquid crystal molecules, the pre-tilt angle of the liquid crystal molecules varies. If the pre-tilt angle of the liquid crystal molecules formed with the photo- or heat-polymerizable monomers being completely polymerized is of relatively large variation, then different light transmittance may be induced in this area, leading to abnormal contrast. After the photo- or heat-polymerizable monomers are polymerized, under the condition of applying no driving voltage or a voltage that is insufficient to cause rotation of the liquid crystal molecules, using a light comprising ultraviolet component or ultraviolet light of the same intensity to irradiate the above liquid crystal material containing the photo- or heat-polymerizable monomers for an extended period of time will similarly lower down the manufacturing efficiency of liquid crystal display devices.
Further, in irradiating the above liquid crystal material containing the photo- or heat-polymerizable monomers with light comprising ultraviolet component or ultraviolet light, since a TFT substrate often comprises portions that may shield the light, the photo- or heat-polymerizable monomers contained in the portion of the liquid crystal material that is not irradiated by the ultraviolet light may not be completely polymerized, and with the migration of the liquid crystal material, this portion of liquid crystal may appear in the display zone. The photo- or heat-polymerizable monomers of this portion show a weak cross-linked structure caused by incomplete polymerization may result in image sticking in a normal display mode.